I think it's because:-
It can spoil your grow area for future grows.
Reduce yield and potency.
Reduce price if growing for sale.
Not just one plant gets pollinated.
You are likely to end up with hundreds of seeds.
Hermaphrodite seeds are likely to produce hermaphrodite offspring.
Its one hell of a bad taste and explosion if you happen to smoke a seed.

Don't get me wrong. It can be a lot of fun to cross breed plants in a controlled environment or feminize seeds for future grows. But accidentally spoiling your otherwise perfect sinsemilla crop demonstrates a degree of skill or planning was lacking.
Just ask a large scale grower who is growing for sale how badly a hermaphrodite can affect their profits.